One of the persistent design constraints in the field of engineering is vibration and/or force impact and/or fatigue management. That is, nearly all engineered devices and systems must embody a design that is sufficiently robust so as to safely survive all movement, vibration, impact, etc. that such a device or system is likely to encounter in its useful life. Examples of such areas of engineered devices and systems are seismic protection devices, construction hardware, seating systems in vehicles such as helicopters, boats, etc., manufacturing equipment and the like, all of which relate to the present invention as will be self-evident from the text below that describes in detail various preferred embodiments.
Such design constraints, however, are no more self evident than in the field of vehicle design. It is well understood that vehicles endure a constant barrage of forces, impacts and vibrations throughout a vehicle's entire useful life. Indeed, when it comes to vehicle design, it may be said that this adverse active environment is perhaps the primary design constraint.
The common and long known methodology for meeting these rigorous design constraints of vehicle design is the use of a spring and damper system located at appropriate locations along the vehicle chassis, most commonly between each tire/wheel assembly and the vehicle frame. The most common type of spring and damper system in this regard is the conventional shock absorber.
Conventional shock absorbers are comprised of two reciprocating cylindrical tubes that extend, via an intervening spring, from the tire/wheel of the vehicle to the vehicle frame. One cylindrical tube is filled with fluid and the other cylindrical tube houses a piston that passes through the fluid when the tubes move relative to each other. When the piston moves, it forces the fluid through restrictive passages within the piston. This thereby controls the speed with which the two tubes can move relative to each other for a given force.
When a vehicle encounters a hole or a bump, the tire/wheel moves in response thereto and thereby tends to urge the spring to either extend or compress. If there was a spring alone (i.e., no cylindrical tubes discussed above) between the tire/wheel and the frame, there is a risk that this difficult terrain will cause the spring to resonate, a condition that adversely effects the handling and ride of the vehicle. The cylindrical tube structure, therefore, substantially inhibits such resonating because movement of the spring is dependent on movement of the two reciprocating tubes. That is, the added resistance to movement of the tubes due to the restrictive flow of fluid resulting from movement of the piston thereby dampens the forces that would otherwise cause the spring to extend or compress. This, in turn, substantially inhibits spring resonance and ensures proper handling and ride of the vehicle.
A number of modifications to this basic shock absorber design have been made over the years in order to enhance the damping effect of the device. For example, changing the size of the restrictive passages in one of the tubes and/or using a fluid with a different viscosity can have material improved effects on the shock absorber performance. Performance characteristics can also be altered by increasing or decreasing the size of the shock, changing the design of the tubes, the internal valving (restrictive passages), etc.
There are, however, practical limits as to how much shock performance may be changed by making such alterations. As a result, alternative damping systems have been formulated.
One such alternative system is based on the utilization of a variable shear strength fluid such as a magnetorheological (MR) fluid. MR fluid based devices are founded on the principle of controlling the shear strength of the MR fluid by inducing and controlling a magnetic field around the piston. Control of this magnetic field can change the shear strength of the MR fluid anywhere from its normal state as a liquid to an energized state that is nearly a solid. Therefore, by precision control of the magnetic field, the shear strength of the MR fluid is adjusted so as to precisely control the damping performance of the device. An example of such an MR device is disclosed in U.S. Pat. No. 6,419,058 which is hereby incorporated by reference in its entirety.
Nonetheless, the demands placed on vehicles, particularly off-road vehicles (as well as other devices and systems that encounter a rugged environment), continues to increase, all with the corresponding demand to avoid any degradation in passenger comfort or endurance. As a result, there is now an expectation and need to provide a damping system that can withstand very sizable range of operating environments, namely, anywhere from a flat, obstruction free surface to the most difficult of off-road conditions. Indeed, the system must not only withstand such environments, but must operate effectively and continuously throughout this wide range of operating environments without degradation in performance.
In this regard, the principle of using MR fluid appears to be well suited to providing the accurate control necessary for the operating environment discussed above. However, the inventors are not aware of any prior art MR devices capable of correctly operating at very high damping forces and/or that support wide ranges of damping forces without the system either encountering undesired cavitation or without being severely damaged. Nor are the inventors aware of any prior art MR devices that have adequate bandwidth for effective isolation of the high frequency road inputs often encountered with difficult terrains.